Automatic telephone systems



A. A. CHUBB ETAL AUTOMATIC TELEPHONE SYSTEMS May 22, 195e 3 Sheets-Sheet 1 Filed Jan. 51. 1952 :AWE

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AUTOMATIC TELEPHONE SYSTEMS 3 Sheets-Sheet 2 Filed Jan. 3l. 1952 //V VENTOES u Y Y E of w Tb. 0 me n ww. A am 0 Y May 22. 1956 A. A. CHUBB am 2,747,019

AUTOMATIC TELEPHONE SYSTEMS Filed Jan. 5l, 1952 3 Sheets-Sheet 3 R66 wel Tufo 67 "W2 TU',

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.4 TTOk/VE Y AUTOMATIC TELEPHONE SYSTEMS Application llauuary 31, 1952, Serial No. 269,153

Claims priority, appiication Great Britain February 2, 1951 Claims. (Cl. 179--18) The present invention relates to automatic switching apparatus for use in automatic exchanges in which a plurality of communication channels is provided by means of a plurality of time interlaced pulse trains, and, in operation, when estalishing communication between two stations connected to the exchange, apparatus associated with a rst station selects a free one of the channels, a signal is transmitted to automatic switching apparatus associated with the second station identifying the selected free channel, and the said automatic switching apparatus serves to connect the second station to the identified free channel. Such automatic exchanges will be referred to hereinafter as automatic exchanges of the kind specified and examples of such exchanges are disclosed in co-pending United States patent applications Nos. 247,072, led September 18, 1951, 257,786, tiled November 23, 1951, and 257,991 and 257,992 tiled November 24, 1951.

The object of the present invention is to provide suitable automatic switching apparatus for use in an automatic exchange of the kind specified to connect the said second station to the channel identified by the channel identifying signal.

According to the present invention automatic switching apparatus for use in an automatic exchange of the kind specied to connect the said second station to the channel identiiied by the channel identifying signal, comprises two or more gate devices, one of the gate devices being for transmitting messages from the irst station to the second station, and the other gate device being for transmitting messages from the second station to the rst station, selector means responsive to the channel identifying signal to select gating pulses in the selected free channel, and connections for applying the gating pulses to the said gate devices whereby communication between the two stations can be established in the selected channel through the two gate devices.

One arrangement in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which Figure 1 is a block schematic diagram of an automatic telephone exchange of the kind specified embodying automatic switching apparatus according to the present invention.

Figure 2 is a circuit diagram of one embodiment of the invention suitable for use in the automatic exchange of Figure 1, and

Figure 3 is a circuit diagram of a part shown in block form in Figure 2.

Figure 1 is a block schematic diagram of an automatic telephone exchange which is fully described in the specitication of co-pending United States patent application No. 257,786. 'Ihis exchange will not therefore be fully described in the present specilication.

The telephone exchange shown in Figure 1 is suitable for use with 2000 subscribers. Of the 2000 subscribers stations connected to the exchange, one is shown at and another at 11, the station 10 being terminated in the exchange at subscribers line circuit 12 and the station 11 being terminated at the subscribers line circuit 13.

The exchange comprises a group of pulse communication channels the pulses in each channel having a recurrence frequency of about 8,000 P. P. S. This group of channels is provided by means including a pulse generator 14 adapted to generate pulses suitable for combination to provide the 100 channels time-interlaced at the pulse repetition frequency of 8,000 P. P. S. in each channel. The 100 channels are terminated by 100 calling units respectively of which one is shown at 15. 100 called units terminate the other ends of the channels respectively when in use, one of the called units being shown at 16. Messages between the calling and called units pass through a GO speech junction 17, a return speech junction 18 or a metering and release junction 19 as the case may be. In the event of a called subscriber being engaged a busy signal is transmitted to the calling subscriber by way of a busy junction 20.

For use in establishing calls between subscribers a number of allotters 24 and registers 23 are provided together with a signalling junction 25 and two further pulse generators 21 and 26.

Assuming the subscriber at the station 10 to be calling the subscriber at the station 11 the rst operation occurs in the subscribers line circuit 12. This contains a finder which linds a free calling unit 15. An allotter 24 then cornes into operation and allots a register 23 to the calling unit 15.

By means of the allotter and a finder in the register the calling unit found by the subscribers line circuit 12 ts connected to the allotted register 23. A dialling tone is then automatically transmitted to the calling subscriber.

The output of the pulse generator 21 is in the form of a recurring sequence of 20 pulses which for conveniece will be divided into groups of ten each. The rst ten pulses in each sequence will be referred to collectively as the d' pulses and the second ten as the u pulses. The ten d' pulses will be referred to individually as the pulses d'0 to d'9 respectively and appear at the terminals Td'n to Td's respectively of the generator 21. The ten u' pulses will be referred to individually as pulses uo to us and appear at the terminals Tuo to Tu9 respectively of the generator 21.

Each of the calling units is connected to one of the terminals Tdo to Td9 and to one of the terminals Tuo to Tus of the generator 2l, different ones of the calling units being connected to different pairs of the terminals Td and Tu. Thus each calling unit is identiiied by a different pair of d and u pulses and, as will be described later, the pair of d and u pulses identifying the calling unit, and hence the selected channel, is transmitted to the called subscribers line circuit to identify the high grade channel on which communication is to be established.

When the calling subscriber hears the dialling tone and dials the number of the called subscriber, the dialling pulses are transmitted through the line circuit 12 and the seized calling unit 1S to the allotted register 23. In the register the four sets of dialling pulses are stored on four uniselectors. Four groups of ten pulses each, which will be referred to as the M, C, D and U pulses are applied to the four uniselectors in the register from the pulse generator 26. Thus the uniselectors in the register select one pulse from each of the four groups of recurring pulses applied thereto and it is arranged that these four selected pulses are combined to form a recurring output pulse which occurs in one recurring channel interval in a recurring sequence of 2,000

annoio channel intervals. ',Ihe sequence may .recur for example at the rate of one per second and the 2,000 channels constitute a group of low grade signalling channels.

The recurring output pulse. from the register isfuSeu as a gate pulse and permits the recurring pair of d', tt' pulses identified with the seized calling unit to pass to a signalling junction 2 5 only in the low grade channel interval -determined by the gating pulse.

The ,output from the signalling, junction is` applied to all subscribers line circuits, These circuits have gating pulses applied thereto however from the pulse generator 26, the gating pulses applied to each subscribers line circuit being in the channel whose number corresponds to the subscribers number on the exchange. Thus the only subscribers line circuit to respondto a pair of d?, u pulses appearing at the output ofthe signalling junction is that one whose gatingA pulse corresponds to the number dialled.

The called subscribersline circuit then hunts for a free called unit and when a free called unit is found this unit functions to select appropriate pulses from the output of the generator 14, to enable the called subscriber to establish communication in the channel terminated by the seized calling unit. The called unit forms the sub ject ot the present invention and one embodiment thereof will now be described in mQre detail with reference to Figures Z-and.

In Figure 2 a control terminal P12 is connected lthrough relay contacts RBS, a relay winding RQ and a battery BATzs to earthI` The terminal P12 is connected to one fixed contact of a control bank of a uniselector forming part of the finder in the called subscribers line circuit. The wiper of this bank is normally earthed. Two terminals +I2 and I2 are connected through relay contacts F2 and F3 respectively and capacitors C22 and C2i to a winding MW2 of a hybrid transformer HY2. An output winding OW2 of the transformer HY2 has one terminal earthed and the other terminal thereof is connected through a capacitor C24 to the control grid of a pentode valve V24 Whose cathode is earthed. The anode of the valve V24 isrconnected through a load resistor R32 to the positive terminal HT|11 of a source (not shown) of D. C. whose negative terminal is earthed. The anode is also connected through a capacitor C25 to an output terminal RSP2 which is connected tothe return speech junction 18 of Figure 1. The screen grid is connected directly to the terminal HT-i-Il. Negative bias is applied to the control grid of the valveV24 from a bias terminal -GB21. The suppressor grid is connected through a capacitor C25 to relay contacts CD1 and throughY a resistor R33 to the negative terminal -GBs of a bias source (not shown).

An input terminal GSPO1 which is connected to the output of the GO speech junction 17 of Figure l is connected through a capacitor C22 to the control grid of apentode valve V25 whose cathode is rearthed. The controlgrid of the pentode V25 is also connected through a resistor R34 to the negative terminal -GBQ of a bias source (not shown) Whose positive terminaly is earthed. The anode of the pentode V25 is connected through the primary winding of a transformer XF2 to the positive terminal HT+12 of a source (not shown) of D. C. whose negative terminal is earthed. The anode is also connectedthrough a capacitoriCzs to the control gridfof a triode .valve V26 whose cathode is earthed. The Yanode of the triode V25 is connected through a relay Winding RB to the positive terminal HT+13 of a source (not shown) of D. C. whose negative terminal is earthed. The control grid of the triode V25 is connected through a rectier W15 to the negative terminal -GBrt of a bias source whose positive terminal is earthed. The screen grid of the pentode V25 is connected through a resistor R35 to the postive terminal HT+12. The suppressor grid is connected through a resistor Ras to the negative terminal -GBio of a bias source (not shown) whose positive terminal is earthed, and through a capacitor C29 to the relay contacts CD1.

The transformer XF2 has two secondary windings S1XF2 and S2XF2. One terminal of the winding SrXFz is connected to the negative terminal -GB12 of a bias source (not shown) whose positive terminal is earthed, and the other terminal thereof is connected through a rectifier Wis to the control grid of a pentode valve V27 Whose cathode is earthed. One terminal of the winding S2XF2 is connected to the negative terminal -GBia of a bias source (not shown) whose positive terminal is carthed, and the other terminall of the winding'SzXFz is connected through a rectiiier W17, and a resistor R37 to the control grid of the pentode V27.

A rectifier W18 is connected between the terminal *GBrz and the left-hand terminal of the Winding S2XF2 in the drawing.

The primary winding ofthe transformer XE2 is tuned by a capacitor C30v whichis shuntedby a resistor R194.

The anode, of the pentode V2ris connected through the input circuitof a lou/pass iilter FIL2 tothepositi'vc terminal HTI14of a sourcetnotshown) of. D. C. whose negative terminal is earthed. The screen grid of, the pentode V27 is connected through a resistor Ras tothe positive terminalrHT-{Flland is decoupled by a capacitor C32, One output terminal of the low-pass filter FILz is connected toY earth andthe other output terminal is connected through the winding IWz of the hybrid transformer HY2 to earth.

The terminal *12; is normally connected to. earth through the relay contacts F3 and a battery BATss, and the terminal -{-l2 is normally connected through the relay contacts F2 and through one winding of a relay F to the moving Contact of relay contacts RB4.' These. contacts are normally open and the fixed. Contact thereof is connected through the` secondary winding ofv a transformer XFs to earth. The primary winding ofthe transformer XF?. is connected to terminals TR to whichring ing current is applied from a suitable source (not shown). The contacts F2 are bridged by a capacitor C54. One terminal of theother winding of relay F is connected through a battery BAT29 to earth and the other terminal thereof is connected through. relay contacts F1 and RBI to earth. One winding of a relay RD is connected between the right-hand plate (in the drawing) of the capacitor C22 and earth. The other winding of the relay RD has one terminal connected to the right-hand plate vof the capacitor C23 and hasthe` other terminal connected through a battery BATso to earth,

A uniselectol ZRU has three banks ZRUl, ZRU2, and ZRU3, eachy of which has ahome contact and bank contacts. The bank ZRUl has ten banks contacts which are connected to the translator 25Lto be described later. The wiper of the bank ZRUI isconnected through a resistor R39 to the controlgrid of a gas-filled triode valve V22. The cathode ofthe valveVze 4is connected to the negative terminal of a bias-source GBM whose positivo terminal is .earthed The anode Aof the `valve Vzais connected through relay contacts 'DAI to. one terminalfof a relay winding-DA. TheV other terminal ofthe Winding DA is connected through relay contacts RQ1 and RB2.i11 parallel tothe positive tcrminal.HT.-}15 of a `source (not shown) ofl D. C. whose negative terminal is earthed.

The bank ZRU2 is a homing bank and has its homing arc BCs connected to earthgthrough'relayrcontacts DAZ. The wiper of the bank ZRU2 is connectedv through contacts ZRUcI'm,. operating windingZRU, a' relay winding CD and a battery BATar-in series toearth.- The home contact of: the bank. ZRU2, ist connected through .relay contacts RQZV to earth.

The bank ZRU3 has tenbank contacts which are connected to ten terminals Tuol to Tue respectively. The wiper of the bankZRUS isI connected through a resistor R40 tothe xedcontact of contacts CD1.

AuniselectorlZRD has threebanks-ZR'DLZRDZ- and ZRDS, each of which has a home Contact and bank contacts. The bank ZRDll has ten bank contacts which are connected to the translatrr 25. The wiper of ZRD1 is connected through a resistor R41 to the control grid of a gas-filled triode V29 whose cathode is connected to earth through a bias source GB15. The anode of the valve V29 is connected through relay contacts UA1 to one terminal of a relay winding UA and the other terminal of the Winding UA is connected through the relay contacts RQll and RBZ to the terminal HT-l-i'.

The bank ZRD2 is a homing bank and the homing arc BC7 is connected through relay contacts UA2 to earth, and the home contact through RQZ to earth. The wiper of the bank ZRDZ is connected through the mechanically operated contact ZFDdm, the operating winding ZRD, the relay winding CD and the battery BAT31 in series to earth.

The bank ZRDS has ten bank contacts which are connected to ten terminals Tdo to Tdg respectively. The wiper of the bank ZRD3 is connected through a rectifier W to the xed contact of contacts CD3..

The moving contact of the contact CD1 in addition to being connected through the capacitors C26 and C29 to the suppressor grids of the pentodes V24 and V25 respectively is connected through a resistor R42 to earth and directly to the moving contact of contacts RDl. The iixed contact of contacts RDl is connected through a resistor R43 to earth and through a capacitor C33 to the control grid of a triode valve V31 Whose cathode is earthed. Negative bias is applied through a resistor R44 to the control grid of the triode V31 from the negative terminal G1310 of a bias source (not shown) whose positive terminal is earthed. The anode of the valve V31 is connected through a capacitor C34 to an output terminal ASP1, and through a resistor R45 to the positive erminal HT-i-ll of a source (not shown) of D. C. whose negative terminal is earthed. The terminal ASP1 is connected to the metering and release junction of Figure 1.

'The translator 25 is connected through relay contacts RQ3 to the positive terminal HT-l-l7 of a source (not shown) of D. C. whose negative terminal is earthed. Ten terminals Tz4'0 to Tu9 and ten terminals Td0 to Td9 are connected to the translator, and a further terminal CS2 is connected to the called subscribers line circuit to receive the pairs of a zt' pulses passed by the called subscribers line circuit.

Referring now to Figure 3, this is a theoretical circuit diagram of the translator 25. The translator cornprises two groups of gas-lled triode valves each group containing ten valves. A rst of the groups contains ten triodes V32 to V41 whose cathodes are connected to earth through resistors R40 and R55 respectively, and directly to the ten bank contacts respectively of the uniselector bank ZRUi. The anodes of the triodes V22 to V41 are connected together and through the relay contacts RQ3 to the terminal HT-l-n. The terminals Tuo to Tu9 are connected through ten rectiiiers W21 to W ten resistors Rss to R75 and ten rectiers W100 to W109 respectively in series to the control grids of the triodes V32 to V41. The terminal CS2 is connected through ten resistors Rss to R95 to the junctions or the rectiiiers and resistors connecting the terminals Tun to Tu9 respectively to the control grids of the triodes V02 to V41. The control grids of the triodes V32 to V41 are connected through capacitors C103 to C112 and resistors R207 to R210 respectively to earth.

The second group contains ten triodes V42 to V51 whose cathodes are earthed through resistors R50 to R05 respectively. The cathodes of these valves are also connected directly to the ten bank contacts respectively of the uniselector bank ZRDl. The anodes of the valves V42 to V51 are connected together and through the relay contacts RQ3 to the terminal HT-l-17. The terminals Tdo to Td'9 are connected through ten rectiers W31 to W40 ten resistors R05 to R85 and ten rectiers W110 to W119 respectively in series to the control grids of the valves V42 to V51. The terminal CS2 is connected through ten resistors R to R105 to the junctions of the rectifiers W31 to W40 and resistors R76 to R05 respectively.

In operation the d and u pulses arriving at the terminal CS2 from the called subscribers line circuit are applied to the control grids of the Valves V32 to V51 through the resistors Rss to R105 and pulse lengthening circuits comprising the rectiers W to W119 and capacitors C103 to C122. The rectiiiers W21 to W40 are, however, conducting to these pulses except the rectiers to which d and u pulses are applied from the terminals Tuo to Tllg simultaneously with the d and u pulses arriving at the terminal CS2. In this example it will be assumed that the pulses d'0 and u'1 appear at the terminal CS2 simultaneously with the pulses d'0 and u1 applied at the terminals Tdo and Tui. Thus the rectiers W31 and W22 are non-conducting in this example and permit the d'0 and u1 pulses applied to the terminal CS2 to pass to the control grids of the valves V42 and V33 respectively. These two valves strike and hence their cathodes become positive.

When the wiper of the bank ZRUl reaches the bank contact connected to the cathode of the valve V33 the positive potential appears at the control grid of the valve V23 (Figure 2) and this valve strikes. Similarly the valve V29 (Figure 2) strikes when the wiper of the bank ZRDI reaches the bank contact connected to the cathode of the valve V42.

Referring to Figure 2, when the valves V22 and V29 strike the relays DA and UA become energised by the anode circuits of these two valves. Thus the contacts DA1, DA2, UA1 and UAZ are operated. The contacts DA1 hold the relay DA and extinguish the valve V28 and the contacts DA2 break the automatic stepping circuit of the uniselector ZRU. The contacts UA1 hold the relay UA and extinguish the valve V29, and the contacts UAZ break the automatic stepping circuit of the uniselector ZRD.

Thus the wipers of the banks ZRD3 and ZRU3 are arrested o-n the bank contacts connected to the terminals Tdt) and Tui respectively. The pulses d0 and u1 are applied to these terminals respectively from the high frequency pulse generator. In the absence of a d0 pulse the rectifier W20 is conducting and hence the u1 pulses are dropped across the resistor R40. The occurrence of a d0 pulse renders the rectier W20 non-conducting and hence the u1 pulse occurring during each d0 pulse passes to the contacts CD1 that is to say pulses in channel No. l. These contacts close simultaneously with the opening of the automatic stepping circuit of the selectors and the pulses in channel No. l pass to the suppressor grids of the valves V24 and V25.

Thus unmodulated pulses of anode current ow in the valves V24 and V25 which are arranged to be normally non-conducting by the bias applied thereto from the terminals -GBs and -GBio respectively. The pulses passed by the valve V25 are applied through the capacitor C20 to the control grid of the valve V26. Thus, pulses of anode current ow in this valve which is arranged to be normally non-conducting by the bias applied thereto from the terminal GB11. Thus the relay RB becomes energised and the contacts RBl and RB4 thereof are operated.

The contacts RBS put an earth on the terminal P12. The contacts RB2 close and hold the relays DA and UA when the relay RQ becomes de-energised on the operation of the contacts RB3.

The contacts RB4 close and apply ringing voltage from the transformer XF3 to the called subscribers station via the terminals +12 and -I2. When the called subscriber answers the relay F becomes energised, the energising circuit being from earth through the battery BAT33 the contacts F3, the called subscribers line, the

contactsFZ, the relaywinding F, the contacts'RB4 andY the secondary' winding of thetransformerXFa back to earth. The contacts RBl'and F1 hold the relay F.

The relay RD then'becomes energised, the energising circuit being from earth through'the battery BATso, one

All pulses from the terminal GSPOi are passed to the control grid of the pentode V25 which is gated by channel No. l pulses as previously described. Thus only the pulsesin channel No. l pass to the transformer XFz.

The primary winding is tuned by the capacitor Cso to a periodicity of approximately twice the width of the pulses applied thereto from the valve V25, and is heavily damped by the resistor R194. The winding S2XF2 hasmore turns than the winding SzXFi and hence provides a greater output voltage than S2XF1.

The terminal -GBis is arranged to be about tive volts less negative than the terminal -GB12.

Assuming the charge in the capacitor C31 to be such that the potential of the upper plate thereof in the drawing is between that of -GBis and -GB12, the rectiers Wis to W19 are non-conducting. When a pulse arrives at the primary winding of the transformer XFz from the Valve V25 the leading edge of the pulse shock excites the transformer. The iirst, and negative, half-cycle of the free oscillation is applied to the anode of Wis which', therefore, remains non-conducting, and to the cathode of W17 which becomes conducting and hence C31 discharges through R37 and Wir until the potential of its upper plate in the drawing equals the potential of the terminal -GB12. Any further negative excursion at the cathode of W11 is damped by W18. Simultaneously W19 becomes conducting and as a result the capacitor C31 discharges further and its upper plate is left at` a potential somewhat below that of the terminal -GB12.

.lust as this negative half-cycle ends, the lagging edge of the pulse arrives and again the transformer XFz is shock-excited but this time in the opposite sense. The first, and positive, half-cycle of this second free oscillation has no effect on W17 and W18. The capacitor C31 is charged, however, through W16 tota value dependent upon the amplitude of the pulse applied to the transformer XFz, Aand lying between the potentials of the terminals -GBiz and -GBi3.

The damping provided by the resistor R194 is arranged to be sufficient to prevent subsequent half-cycles from aiecting the charge 'm the capacitor C31.

Thus the capacitor C31 presents relatively broad pulses to the valve V27 in response to relatively narrow pulses passedk by the valve V25, the amplitude of the broad pulses being dependent upon the amplitude of the narrow pulses.

The broad amplitude-modulated pulses are demodulated by being pressed through the low-pass filter F112 and the speech voltages are passed through the transformer HY2, the capacitors C22 and C23, the contacts F2 and F3, and the called subscribers line circuit to the called subscribers une.

Speech voltages from the called subscriber are passed through his line circuit, and the hybrid transformer HY2 to the control grid of the pentode Valve V24. Here they serve to amplitude-modulate the pulses (in channel No. l) applied from the anode of V24 to the terminal RSP2. These pulses are applied through the Return speech junction to the calling unit.

When a call is ended and the calling subscriber replaces his handset on its rest it is arranged that the calling unit is released and hence pulses arriving at the terminal GSPO1 of Figure 2 cease and the relay RB becomes deenergised. The' contactsV RBI reopen' and release the relayY F. Thefcontacts RBZfopen and release-the relays DA and UA. Thus the contacts DA2 and UAZ'closei The'called and the uniselectors ZRU and ZRD home; unit is then ready for use in making another call.

Although an arrangement has been described inr whichr two pulses (d, u) arey transmitted to the called unit in other systems a diierent number of pulses may be-transmitted. For example `where more than 100 channels are' provided it will be necessary to transmit three pulses, one of which is representative ofthe ls, the second of the lOs and the third of the units in the selected channel number` A further group of ten tubes is then added in Figure 3 and appropriate'gating pulsesv applied thereto. Likewise a further uniselector is added in Figure 2 and controlled by the tubes in the third group in Figure-3.

We claim:

l. ln combination, a selector device having a plurality of input terminals, an output terminal and'selector means for selectively connecting the input terminals to saidoutput terminal, means to apply pulse trains of different phases to said input terminals respectively, a control circuit to actuate said selector means, a source of control voltage connected to said control circuit to cause said selector means to select one of said input terminals for connection to said output terminal, a pulse demodulator, a pulse modulator and means to connect said outputl terminal to said demodulator and modulator.

2. ln combination, a selector device having a plurality of input terminals, an output terminal and selector means for selectively connecting the input terminals to saidoutput terminal, means to apply pulse trains having different channel identifying signals to said input terminals respectively, a control circuit to actuate said selector means, a source of control voltage connected to said control circuit to cause said selector means to select one of said'nput terminals forconnection to said output terminal, a pulse demodulator, a pulse modulator and means to connect said output terminal to said demodulator and modulator.

3. A combination as set forth in claim 2 wherein each channel identifying signal includes a pulse whose instant of occurrence identities a different pulse train, and wherein the selector means comprises a group of gas lilled electron discharge tubes each of which has an anode, a cathode andV a control electrode, a plurality of gate devices in the control electrode circuits of the several discharge tubes respectively, means for causingv said gate devices to open in turn, and circuit means for applying a selected one of said channel identifying signals to the control electrodes of all said discharge tubes through said gate devices whereby the pulse of the said selected channel identifying signal passes to the control electrode of only one of said discharge tubes to strike said tube and identify a selected pulse train..

4. ln combination, a selector device having a plurality of input terminals, an output terminal and selectormeans for selectively connectingthe input terminals to said output terminal, means to apply pulse trains of diterent phases to said input terminals respectively, a control circuit to actuate said selector means, a source of control signals connected to said control circuit to cause said selector means to select one of said input terminals for connection to said output terminal, a pulse demodulator, a pulse modulator, and. means to connect said output terminal to said demodulator and modulator.

5. A combination as set forth in claim 4, wherein each control signal includes a pulse, the instants of occurrence of pulses in different ones of said control signals identifying different ones of said pulse trains, and wherein the selector means comprises a group of gas-filled electron discharge tubes each having an anode, a cathode, and a control electrode, a plurality of gate devices in the con'- trol electrode circuits of the discharge tubes respectively", means for causing said gate devices'to open in turn, and circuit means for applying a selected one of said'control signals to all said gate devices whereby the pulse of the selected control signal passes to the control electrode of only one of said discharge tubes to strike said tube and identify one of the said pulse trains io be selected.

References Cited in the le of this patent UNITED STATES PATENTS Hubbard July 3, 1945 Ransom May 3, 1950 10 

